1. Field of the Invention
The present invention relates generally to integrated circuit manufacture and more particularly to a method of and apparatus for monitoring and controlling atmospheric pressure in a semiconductor processing facility.
2. Description of the Related Art
One trend in modern integrated circuit manufacture is to produce electronic devices on semiconductor substrates having feature sizes that are as small as possible. To produce a high density integrated circuit efficiently, semiconductor processes include the production of complex circuits on a single monolithic substrate, thereby allowing relatively large circuit systems to be incorporated on a single and relatively small die area. Further, many such die are commonly produced on a single wafer which, after production, is diced into a plurality of integrated circuits.
The benefits of high density circuits can only be realized if advanced processing techniques are used. For example, semiconductor process engineers and researchers often study the benefits of electron beam lithography and x-ray lithography to achieve the higher resolutions needed for submicron features. To some extent, wet etch has given way to a more advanced anisotropic (dry etch) technique. Each of these complex processes requires specialized equipment. Moreover, some processes even require specialized rooms (zones) or tools having specified environmental conditions. The specified environmental conditions include temperature, air quality (highly filtered air), humidity and atmospheric air pressure.
One reason it is very important to carefully monitor and control air pressure in the various zones and tools that are used in the fabrication process is to control the flow of impurities. The differential air pressure must be controlled between zones so as to prevent contamination flow to the more critical rooms or zones. Impurities are known to flow from a room having a higher atmospheric pressure to a room having lower atmospheric pressure. Thus, rooms are often kept at different atmospheric pressure levels relative to each other according to the importance of maintaining that room's (zone's) cleanliness. For example, a photolithographic zone must be kept free from contaminants introduced by the main fabrication room.
Similarly, the photolithographic zone and the main fabrication room should be kept at an air pressure level that precludes the flow of impurities from the polish zone (the dirtiest of the three zones). Therefore, the differential air pressure in the various rooms or zones must be monitored and controlled to prevent the flow of impurities from zone to zone.
In addition to monitoring differential air pressure in the various processing rooms or zones, differential air pressure is also often monitored for various pieces of equipment or tools that are used in the semiconductor manufacturing processes. However, it is difficult to consistently use the same reference point in calculating differential air pressures. For example, it is not desirable to merely use the air pressure of the room in which the machine resides as a reference pressure. One reason, as discussed before, is that the various rooms are maintained at different air pressure levels. Moreover, the air pressure levels in the rooms can fluctuate. If an outside pressure is used, merely having a wind on one side of a building can cause it to have a higher atmospheric pressure on the windward side due to air pressure that may result from open doors or windows. As a result, one possible reference is to use the air pressure in an isolated room that is not used on a routine basis. By using such a room as a pressure reference, one can more effectively measure differential pressures through out a building. For example, a photolithographic zone is often kept at an atmospheric pressure that results in a differential static pressure set point that is equal to 0.1 inches of water in a water column. The main fabrication room is kept at a static pressure set point that is equal to 0.08 inches of water and the polishing zone is kept at a static pressure set point that is equal to 0.06 inches of water. Each of these static pressure set points is greater than the atmospheric pressure of an external reference. These slightly positive pressures relative to an outside reference are necessary to control the influx of contaminants. Because controlling air flow and air pressure are the primary methods of maintaining cleanliness in the fabrication facility, the proper levels of air pressure must be maintained to control air flow.
One difficulty in using a reference room for making differential pressure measurements is that it is necessary, in order to make real-time differential pressure measurements, to install and route a hose or a pipe (piping) from there to the point where a measurement is to be taken. Having to install piping presents many problems. First, is often necessary to route the piping long distances around or through obstacles. In some cases, is necessary to develop a hole in a wall of a room, such as the photolithographic zone, in which it is important to minimize the ways in which impurities may be introduced. Second, having to build a piping system to allow the measurements to be taken relative to an external reference room prevents flexibility because a meter has to be connected to the piping. Accordingly, it is not possible to take random measurements in various locations in the fabrication facilities. This lack of flexibility makes troubleshooting for potential problems in a fabrication process difficult.
One common piece of equipment that is used to monitor and control pressure is the manometer. A typical manometer includes two ports, namely, a high port and a low port, each being adapted to receive a conduit for carrying a fluid pressure. The value displayed on manometer display is the difference in pressure between the high port and the low port. A typical air pressure controller or manometer compares the difference in pressures of each area to be monitored in relation to the reference pressure. The controller also modulates devices known as dampers to maintain the desired setpoint. The modulated areas are maintained at a slightly positive pressure relative to outside to prevent "dirty air" from entering the building. Each modulated area is also pressurized, as discussed above, with respect to each other to control the flow of contaminants in the various zones, rooms or tools. As discussed previously, however, taking real time differential measurements are difficult using current equipment and systems.
One difficulty in taking real time differential measurements is that current systems require the routing of tubing or piping for carrying fluid pressures throughout the fabrication facility. Having to route tubing through a fabrication facility to allow real time differential pressure measurements can cause safety problems and can introduce impurities into a clean environment. Impurities can be introduced because long sections of tubing cannot be thoroughly cleaned internally. Moreover, penetrating walls to create apertures for the tubing can also cause leakage of conditioned and filtered air. Thus, there exists a need in the art for an improved system and method for measuring differential air pressures and for controlling the air pressure in the various rooms, zones and pieces of equipment.